1. Field of the Invention
The present invention is directed to the disposal of organic wastes through composting and, more specifically, to composting systems for large scale treatment of industrial and municipal wastes.
2. Discussion of the Related Art
Composting is a biological process of decomposition. Given adequate time and the proper environmental conditions, microorganisms turn raw organic matter into stabilized products. The products of composting include carbon dioxide, water, and a complex form of organic matter called compost, which is especially useful as a soil amendment. The key parameters in composting process management are the available carbon to nitrogen ratio, the moisture content, the oxygen content, and the temperature of the composting material.
Carbon serves primarily as a food source for the microorganisms involved in composting. Nitrogen is the primary constituent of protein which forms over 50% of dry bacterial cell mass and is, therefore, necessary for protein synthesis and the optimal growth of microbial populations in the compost pile. It is well known in the art that the preferred carbon-to-nitrogen ratio for composting is about 30 parts carbon for each part nitrogen by weight (30:1). At lower ratios the excess nitrogen supplied will be lost in the form of mobile nitrogen compounds, such as ammonia gas, and can cause undesirable odors or other environmental problems. Higher carbon-to-nitrogen ratios result in an insufficient supply of nitrogen for optimal microbial population growth resulting in a slow rate of degradation. The carbon-to-nitrogen ratio can be increased through the addition of materials high in carbon, such as fallen leaves, straw, woodchips, sawdust, bark, paper, and cardboard. The carbon-to-nitrogen ratio can be decreased by the addition of materials high in nitrogen, such as vegetables, coffee grounds, grass clippings, and manure or sewage.
It must be noted, however, that the carbon-to-nitrogen ratio is a useful measurement only when the degree of biological availability of the carbon is taken into account. Because microbial activity takes place on the surface of the composting material, the effect of the carbon-to-nitrogen ratio of a carbon rich material can be magnified by increasing the surface area of the material, such as by grinding or shredding. Furthermore, some carbon-rich materials, such as newspapers and straw, contain cellulose fibers that are highly resistant to microbial action. Although degradation will occur at relatively slow rates in these materials, chemical pretreatment can increase the degradation of these resistive materials.
Moisture content is another key environmental parameter of composting material. Microbially induced decomposition occurs most rapidly in liquid films found on the surfaces of organic particles. Whereas, inadequate moisture content inhibits bacterial activity, excess moisture content can inhibit the aerobic process. Excessive moisture content results in anaerobic microbial activity, as well as nutrient leaching. This anaerobic activity can produce undesirable odorous compounds, such as hydrogen sulfide. The moisture content of a composting pile is typically related to the carbon-to-nitrogen ratio in that degradable materials that are high in carbon are correspondingly low in moisture. Whereas, materials that are high in nitrogen are typically high in moisture. However, as the composting process completes the mesophilic stage (0-40.degree. C.) and enters the thermophilic stage (40-60.degree. C.), the heat produced can result in the evaporation of a significant amount of the resident moisture.
Excessive moisture content can also result in the leaching of essential nutrients from the composting pile, including phosphorous, potassium, and other trace minerals, which are essential to microbial metabolism. Although these nutrients are not normally limiting, they must be present in adequate supplies for microbial activity.
Oxygen content and temperature are important environmental parameters of composting that fluctuate in response to microbial activity which consumes oxygen and generates heat. As microorganisms oxidize carbon for energy, oxygen is used up, and carbon dioxide is produced. Without sufficient oxygen the process will become anaerobic and produce odorous compounds. Oxygen content is also linked to moisture content in that excessive moisture content can reduce the available oxygen supply resulting in anaerobic pockets within the composting pile.
The temperature of the composting pile varies according to the type and size of the microorganism community resident therein. Mesophilic microorganisms are dominant from the initial stage of decomposition until the temperature rises above about 40.degree. C. and rapidly breakdown the soluble, readily degradable compounds. The mesophilic microorganisms become less competitive as the temperature rises above about 40.degree. C., and thermophilic microorganisms take over. Current Environmental Protection Agency regulations require temperatures in excess of 55.degree. C. (131.degree. F.) for several days to destroy pathogens within the composting pile. Because temperatures over about 65.degree. C. significantly reduce microbial populations and limit decomposition rates, the ideal operating temperature range for a composting pile is relatively narrow.
There are several different reasons why composting remains an invaluable practice. Yard and food wastes make up approximately 30% of the waste stream in the United States. Composting most of these waste streams would reduce the amount of municipal solid waste requiring disposal by almost one fourth, while at the same time provide a nutrient-rich soil amendment.
As composting has become increasingly popular in recent years as a means for recycling a variety of organic materials as part of municipal and industrial solid waste management programs, various composting technologies have been or are being developed. These technologies include, for example, static pile composting, windrow composting, aerated windrow composting, and in-vessel composting employing horizontal agitated bay reactors and vertical reactors. In all such systems, cost effectiveness and automation are desirable. Regarding cost, reducing the space required for a given throughput of composting material is a well recognized need in the industry. Composting operations employing windows, for example, are thought to have an undesirably low ratio of composting materials throughput to processing area square footage, while in-vessel and closed reactors compost material may typically be mounded 20 to 30 feet high. This mounding, however, produces technical difficulties regarding the adequacy of aeration in the reactor vessel leading, in some cases, to unacceptably large pockets of anaerobic activity within the pile. This anaerobic activity leads to the equally undesirable need for removal of odorous compounds from the reactor vessel environment before exhausting it to the atmosphere.
Certain in-vessel composting systems, particularly those comprising open bays within a building, have been used with excellent results. One system of this type employs automated agitators to thoroughly mix and aerate composting material in parallel bays. Starting at the discharge end of an open elongated composting bay, the agitator moves through the bed of composting material toward the loading end of the bay. Typically, the agitator travels through each bay mixing the material and rearwardly displacing it from the loading end toward the discharge end. As the agitator progresses through the bay, a moveable member repeatedly repositions itself in the exhaust stream such that the distance of rearward displacement of composting material is gradually increased to accommodate material which has had progressively less residence time in the bay and, accordingly, has experienced less reduction in the volume due to decomposition and moisture content reduction. An agitator of this type is described by Hagen et al., in U.S. Pat. No. 5,387,036 which is incorporated herein in its entirety by reference. Agitators of this type generally comprise a feeder, typically a rotating toothed drum, and a conveyor. In operation, the rotating drum takes composting material from the bed forward of the agitator and feeds it rearwardly to the conveyor, which in turn discharges the material aft of the agitator. As discussed above, the discharge is regulated as it is rearwardly displaced by the conveyor to produce a bed of substantially uniform depth aft of the agitator as it passes through the bay.
As demands on municipal composting systems increase, the capability of treating larger volumes of composting material in relatively small reaction vessels becomes increasingly desirable. Accordingly, existing systems having a plurality of open horizontal bays, typically between about 6 and 10 feet wide and up to about 300 feet long have been excessively loaded forming composting beds of increasing heights. The plurality of open bays are typically placed side by side and can be served by a single agitator. These systems can be used to compost a wide variety of materials, and the composting rate can typically be regulated to meet varying demand. However, when these large bays are at or near full capacity, the action of the rotating drum of the agitator tends to burrow through the composting pile as the agitator progresses through the bay. As the burrowing action continues, the weight of the undisplaced composting material can become sufficiently great to allow large quantities to suddenly collapse onto the rotating drum, thus slowing its rotation and, consequently, the agitator's progression through the bay. If a sufficient quantity of composting material collapses, the rotating mechanism can stall, resulting in costly delays while the unit is stopped, reversed, cleaned and/or repaired.
Furthermore, in a typical large scale composting operation, air contact along with the rising heat produced by microbial action combine to dry the upper portion of the composting bed, forming a crust-like layer of composting material near the top. As the agitator travels through the bay, large sections of the crust remain intact for extended periods and then, suddenly, crack and fall onto the rotating drum, which increases the stalling frequency of the rotating mechanism.
Attempts to obviate this problem include increasing the diameter of the rotating drum to approximately the maximum height capacity of the bay. This has proven to be an untenable solution, however, because of the added materials costs involved in manufacturing the larger drums coupled with the added energy costs involved in rotating the drum. Furthermore, the use of larger drums adds to the overall size and weight of the agitator resulting in additional problems, including making the transportation of the agitator between bays by existing equipment more difficult.
As the demand on the present municipal and industrial composting systems continues to grow, new and improved methods of increasing the output of existing facilities are needed to supply the market. However, as discussed above, certain critical environmental parameters must be maintained in the composting pile to ensure efficient, environmentally sound degradation of composting materials.